The retention of juvenile characteristics in the adults of a species. Humans, for example, are sometimes said to demonstrate neoteny by retaining through adulthood the relatively large head and hairlessness characteristic of very young primates. The body proportions of flightless birds, which resemble those of fetal flying birds, are also considered to be evidence of neoteny.

The attainment of sexual maturity by an organism still in its larval stage, seen in certain amphibians and insects. Certain species of salamanders, for instance, demonstrate neoteny as they become sexually mature but remain aquatic and do not develop legs. Neoteny sometimes occurs in response to environmental stresses such as low temperature or lack of iodine (which is essential for the thyroid gland). If environmental conditions improve, the organism can often develop into a fully mature adult form.